Day of Week Calculator

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How it works

The weekday is read from the calendar itself: the date is converted to a single day count (days since a fixed reference date), and the weekday cycles every 7 days from that anchor. That is why the tool works equally well for a date in 1950, today, or 2100 — the 7-day week has run unbroken through all of them.

Day of the year counts from January 1 as day 1. In a leap year, every date after February 29 sits one higher than in a common year. Days remaining is simply the days in the year (365 or 366) minus the day of the year.

ISO week number follows the ISO 8601 standard used in business and logistics: weeks start on Monday, and week 1 is the week containing the year's first Thursday. That is why a date like January 1 can belong to week 52 or 53 of the previous ISO year, and late-December dates can belong to week 1 of the next one — the tool shows the ISO week-numbering year alongside the week so there is no ambiguity.

Common uses: checking what weekday a birthday, anniversary, or historical date fell on, planning future events, verifying a weekday printed on an old document, and finding the week number for project and payroll schedules.

Frequently asked questions

Why does January 1 sometimes show week 52? Under ISO rules a week belongs to the year that contains its Thursday. If January 1 is a Friday, Saturday, or Sunday, its week still contains the previous year's last Thursday, so it is numbered in that year.

How far back and forward does this work? For any date the picker accepts. Weekday results use the proleptic Gregorian calendar — the modern calendar extended backwards — which is the standard convention for date tools; dates before the Gregorian calendar's 1582 introduction were recorded under the Julian calendar in many countries.

What is a leap year? A year divisible by 4, except century years, which must be divisible by 400. So 2000 was a leap year, 1900 was not, and 2028 will be.